package org.apache.lucene.search;

/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import java.io.Closeable;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.TimeUnit;

import org.apache.lucene.index.IndexReader; // javadocs
import org.apache.lucene.search.NRTManager; // javadocs
import org.apache.lucene.search.IndexSearcher; // javadocs
import org.apache.lucene.store.AlreadyClosedException;
import org.apache.lucene.util.IOUtils;

/**
 * Keeps track of current plus old IndexSearchers, closing
 * the old ones once they have timed out.
 *
 * Use it like this:
 *
 * <pre>
 *   SearcherLifetimeManager mgr = new SearcherLifetimeManager();
 * </pre>
 *
 * Per search-request, if it's a "new" search request, then
 * obtain the latest searcher you have (for example, by
 * using {@link SearcherManager} or {@link NRTManager}), and
 * then record this searcher:
 *
 * <pre>
 *   // Record the current searcher, and save the returend
 *   // token into user's search results (eg as a  hidden
 *   // HTML form field):
 *   long token = mgr.record(searcher);
 * </pre>
 *
 * When a follow-up search arrives, for example the user
 * clicks next page, drills down/up, etc., take the token
 * that you saved from the previous search and:
 *
 * <pre>
 *   // If possible, obtain the same searcher as the last
 *   // search:
 *   IndexSearcher searcher = mgr.acquire(token);
 *   if (searcher != null) {
 *     // Searcher is still here
 *     try {
 *       // do searching...
 *     } finally {
 *       mgr.release(searcher);
 *       // Do not use searcher after this!
 *       searcher = null;
 *     }
 *   } else {
 *     // Searcher was pruned -- notify user session timed
 *     // out, or, pull fresh searcher again
 *   }
 * </pre>
 *
 * Finally, in a separate thread, ideally the same thread
 * that's periodically reopening your searchers, you should
 * periodically prune old searchers:
 *
 * <pre>
 *   mgr.prune(new PruneByAge(600.0));
 * </pre>
 *
 * <p><b>NOTE</b>: keeping many searchers around means
 * you'll use more resources (open files, RAM) than a single
 * searcher.  However, as long as you are using {@link
 * IndexReader#openIfChanged}, the searchers will usually
 * share almost all segments and the added resource usage is
 * contained.  When a large merge has completed, and
 * you reopen, because that is a large change, the new
 * searcher will use higher additional RAM than other
 * searchers; but large merges don't complete very often and
 * it's unlikely you'll hit two of them in your expiration
 * window.  Still you should budget plenty of heap in the
 * JVM to have a good safety margin.
 * 
 * @lucene.experimental
 */

public class SearcherLifetimeManager implements Closeable {

    private static class SearcherTracker implements Comparable<SearcherTracker>, Closeable {
        public final IndexSearcher searcher;
        public final long recordTimeSec;
        public final long version;

        public SearcherTracker(IndexSearcher searcher) {
            this.searcher = searcher;
            version = searcher.getIndexReader().getVersion();
            searcher.getIndexReader().incRef();
            // Use nanoTime not currentTimeMillis since it [in
            // theory] reduces risk from clock shift
            recordTimeSec = TimeUnit.NANOSECONDS.toSeconds(System.nanoTime());
        }

        // Newer searchers are sort before older ones:
        public int compareTo(SearcherTracker other) {
            // Be defensive: cannot subtract since it could
            // technically overflow long, though, we'd never hit
            // that in practice:
            if (recordTimeSec < other.recordTimeSec) {
                return 1;
            } else if (other.recordTimeSec < recordTimeSec) {
                return -1;
            } else {
                return 0;
            }
        }

        public synchronized void close() throws IOException {
            searcher.getIndexReader().decRef();
        }
    }

    private volatile boolean closed;

    // TODO: we could get by w/ just a "set"; need to have
    // Tracker hash by its version and have compareTo(Long)
    // compare to its version
    private final ConcurrentHashMap<Long, SearcherTracker> searchers = new ConcurrentHashMap<Long, SearcherTracker>();

    private void ensureOpen() {
        if (closed) {
            throw new AlreadyClosedException("this SearcherLifetimeManager instance is closed");
        }
    }

    /** Records that you are now using this IndexSearcher.
     *  Always call this when you've obtained a possibly new
     *  {@link IndexSearcher}, for example from one of the
     *  <code>get</code> methods in {@link NRTManager} or {@link
     *  SearcherManager}.  It's fine if you already passed the
     *  same searcher to this method before.
     *
     *  <p>This returns the long token that you can later pass
     *  to {@link #acquire} to retrieve the same IndexSearcher.
     *  You should record this long token in the search results
     *  sent to your user, such that if the user performs a
     *  follow-on action (clicks next page, drills down, etc.)
     *  the token is returned. */
    public long record(IndexSearcher searcher) throws IOException {
        ensureOpen();
        // TODO: we don't have to use IR.getVersion to track;
        // could be risky (if it's buggy); we could get better
        // bug isolation if we assign our own private ID:
        final long version = searcher.getIndexReader().getVersion();
        SearcherTracker tracker = searchers.get(version);
        if (tracker == null) {
            tracker = new SearcherTracker(searcher);
            if (searchers.putIfAbsent(version, tracker) != null) {
                // Another thread beat us -- must decRef to undo
                // incRef done by SearcherTracker ctor:
                tracker.close();
            }
        } else if (tracker.searcher != searcher) {
            throw new IllegalArgumentException("the provided searcher has the same underlying reader version yet the searcher instance differs from before (new=" + searcher + " vs old=" + tracker.searcher);
        }

        return version;
    }

    /** Retrieve a previously recorded {@link IndexSearcher}, if it
     *  has not yet been closed
     *
     *  <p><b>NOTE</b>: this may return null when the
     *  requested searcher has already timed out.  When this
     *  happens you should notify your user that their session
     *  timed out and that they'll have to restart their
     *  search.
     *
     *  <p>If this returns a non-null result, you must match
     *  later call {@link #release} on this searcher, best
     *  from a finally clause. */
    public IndexSearcher acquire(long version) {
        ensureOpen();
        final SearcherTracker tracker = searchers.get(version);
        if (tracker != null && tracker.searcher.getIndexReader().tryIncRef()) {
            return tracker.searcher;
        }

        return null;
    }

    /** Release a searcher previously obtained from {@link
     *  #acquire}.
     * 
     * <p><b>NOTE</b>: it's fine to call this after close. */
    public void release(IndexSearcher s) throws IOException {
        s.getIndexReader().decRef();
    }

    /** See {@link #prune}. */
    public interface Pruner {
        /** Return true if this searcher should be removed. 
         *  @param ageSec how long ago this searcher was
         *         recorded vs the most recently recorded
         *         searcher
         *  @param searcher Searcher
         **/
        public boolean doPrune(int ageSec, IndexSearcher searcher);
    }

    /** Simple pruner that drops any searcher older by
     *  more than the specified seconds, than the newest
     *  searcher. */
    public final static class PruneByAge implements Pruner {
        private final int maxAgeSec;

        public PruneByAge(int maxAgeSec) {
            if (maxAgeSec < 1) {
                throw new IllegalArgumentException("maxAgeSec must be > 0 (got " + maxAgeSec + ")");
            }
            this.maxAgeSec = maxAgeSec;
        }

        public boolean doPrune(int ageSec, IndexSearcher searcher) {
            return ageSec > maxAgeSec;
        }
    }

    /** Calls provided {@link Pruner} to prune entries.  The
     *  entries are passed to the Pruner in sorted (newest to
     *  oldest IndexSearcher) order.
     * 
     *  <p><b>NOTE</b>: you must peridiocally call this, ideally
     *  from the same background thread that opens new
     *  searchers. */
    public synchronized void prune(Pruner pruner) throws IOException {
        // Cannot just pass searchers.values() to ArrayList ctor
        // (not thread-safe since the values can change while
        // ArrayList is init'ing itself); must instead iterate
        // ourselves:
        final List<SearcherTracker> trackers = new ArrayList<SearcherTracker>();
        for (SearcherTracker tracker : searchers.values()) {
            trackers.add(tracker);
        }
        Collections.sort(trackers);
        final long newestSec = trackers.isEmpty() ? 0L : trackers.get(0).recordTimeSec;
        for (SearcherTracker tracker : trackers) {
            final int ageSec = (int) (newestSec - tracker.recordTimeSec);
            assert ageSec >= 0;
            if (pruner.doPrune(ageSec, tracker.searcher)) {
                searchers.remove(tracker.version);
                tracker.close();
            }
        }
    }

    /** Close this to future searching; any searches still in
     *  process in other threads won't be affected, and they
     *  should still call {@link #release} after they are
     *  done.
     *
     *  <p><b>NOTE: you must ensure no other threads are
     *  calling {@link #record} while you call close();
     *  otherwise it's possible not all searcher references
     *  will be freed. */
    public synchronized void close() throws IOException {
        closed = true;
        final List<SearcherTracker> toClose = new ArrayList<SearcherTracker>(searchers.values());

        // Remove up front in case exc below, so we don't
        // over-decRef on double-close:
        for (SearcherTracker tracker : toClose) {
            searchers.remove(tracker.version);
        }

        IOUtils.close(toClose);

        // Make some effort to catch mis-use:
        if (searchers.size() != 0) {
            throw new IllegalStateException("another thread called record while this SearcherLifetimeManager instance was being closed; not all searchers were closed");
        }
    }
}
